20 - Interactive effects of freezing and drought on long distance transport: A case study of chaparral shrubs of california

Abstract

Chaparral species are evergreen and retain metabolically functional leaves throughout the year, requiring uninterrupted supplies of water to remain hydrated. Water stress-induced or freeze-thaw-induced embolism of xylem water conduits represents a permanent loss of hydraulic capacity, as there is no evidence of reversal mechanism in adult shrubs. Assuming that resistance to freezing- or drought-induced injury incurs some cost, one might surmise that leaves should be about equal in susceptibility to injury by water stress and freezing as xylem conduits are to water stress and freezing-caused embolism. In C. crassifolius, stem xylem is more susceptible to water stress than are leaves (whole branchlet dieback with no recovery), whereas in the chaparral shrub Quercus berberidifolia leaves are more susceptible than stem xylem (entire leaf canopy dies in response to severe drought and remains on a shrub until new leaf emergence on stems with winter rains). In R. ovata, stem xylem is more susceptible to freeze-caused dysfunction than leaves; however, in a closely related species, M. laurina, leaves and stems are equally susceptible to freezing injury. Thus the assumption of equal susceptibility to water stress and freezing between leaves and stem xylem does not always hold. Other survival traits may compensate for imbalances in the susceptibility of leaves and stem xylem, such as the high resistance to water loss in leaves of R. ovata that allows leaves to persist in a partially hydrated state until the vascular cambium produces new xylem.